I want the height of the iframe to match the height of the content.php file. Instead I get a scrollbar. The height of the content.php file is always different, so I cannot give the iframe a fixed height.

Why not use wordpress ( http://wordpress.org ) ?
There are control panels available but only in html&css&js (so no functionality) because each site is unique and needs it’s own cms. You’d have to code the cms yourself.

It would be great if you could create a text editor with a clean interface like coda, textmate, and espresso have. There aren’t any cool text editors available for Windows. I use e-texteditor myself, but I don’t really like it. People tend to switch from pc to mac just because they then have proper text editing software. For me, $2000 is a little too much, but I’m willing to spend up to $100 on a proper text editor.

If you focus on the interface, you’ll get a lot of sales It should definately support code-coloring and perhaps snippets and ftp. I know that this is a LOT of work, but it’s just a suggestion

I will explain how it works on that site:
There a colored image, and a canvas element. When the page is loaded, the image is drawn in canvas but desaturated. The original image is hidden. When you then hover over the canvas element, the colored image fades in.

This is a smart technique, but it can be done much simpler with just 2 images: one black&white and one in color.